Charge pump with charge injection

ABSTRACT

The invention relates to a charge pump.  
     The charge pump comprises n cells in series, each cell comprising a shift capacity (Cdi) and a storage capacity (Cmi), respectively having a shift parasitic capacity (Cpi) and a storage parasitic capacity (Cqi). The presence of the parasitic capacities (Cpi, Cqi) leads to a loss of charge transmitted to the cell. Each cell comprises charge injection means (Cdinji, Cminji) for partially or totally compensating, or even overcompensating, the lost charge quantity.  
     The charge pump according to the invention finds a particularly advantageous application when it is implemented in silicon-on-insulator technology.

TECHNICAL FIELD AND PRIOR ART

[0001] The present invention relates to a charge pump.

[0002] A charge pump is an electronic device for transferring charge, permitting the absolute value of an electrical potential to be increased. It is a DC-DC converter which provides a DC voltage from a switched low voltage DC source.

[0003] Charge pumps are used in low-power applications, typically below 1 W. They are found, for example, in voltage step-up devices (voltage doublers, voltage triplers, etc.), low-power voltage inverters, television THT blocks (10-20 kV), DC-DC converters with discrete elements, integrated circuits for mobile telephones (EEPROM circuits)(EEPROM for Electrically Erasable Programmable Read-Only Memory), etc. Electromagnetic converters (transformers) replace them for high powers, typically greater than 1 W.

[0004] The schematic circuit diagram of a charge pump according to the prior art is shown in FIG. 1.

[0005] A charge pump comprises an interrupter I controlled by a signal K and n identical cells in series. The number n is equal to 5 here. Generally, the number n is typically less than or equal to 10. The interrupter I switches a DC supply voltage Uo to form a switched voltage Uc. Each cell shifts the electric potential between its input and its output by a voltage value of amplitude equal, in a first approximation, to the absolute value |Uo| of the supply voltage Uo. Ideally, the output voltage Us of the charge pump is given by:

Us=n×Uo

[0006] A cell of rank i (i=1, 2, . . . , n) is constituted by two switches D1 i and D2 i and two capacities Cdi and Cmi, the capacity Cdi acting to shift the voltage entering the cell and the capacity Cmi acting to store this voltage. Each component is subjected to a potential difference equal at most to the supply voltage Uo. The switches D1 i and D2 i are generally formed by diodes. The diodes advantageously permit a simplicity of use, because they do not need a control signal. According to certain embodiments, however, the switches can likewise be formed by means of interrupters.

[0007]FIG. 2 shows the actual circuit diagram of the charge pump whose schematic diagram is given in FIG. 1.

[0008] The capacities Cdi and Cmi of the cell of rank i have respective parasitic capacities Cqi and Cpi. The parasitic capacities Cqi and Cpi cause a loss of the charge transmitted to the cell. This loss of charge results in greatly limiting the voltage delivered at the output of the charge pump. As has been mentioned above, the maximum number of cells of a charge pump according to the prior art is typically less than or equal to 10. Beyond 10 cells, in fact, the loss of charge becomes so great that it is no longer possible to design a correctly operating charge pump.

[0009] The invention does not have these disadvantages.

SUMMARY OF THE INVENTION

[0010] In fact, the invention relates to a charge pump comprising a block of n cells in series, each cell comprising a shift capacity (Cdi) and a storage capacity (Cmi), the voltage applied at the input of the block of n cells being a switched voltage (Uc) formed from a DC supply voltage (Uo), the shift capacity (Cdi) having a shift parasitic capacity (Cpi) formed between a first plate of the shift capacitor and a reference conductor, and the storage capacity (Cmi) having a storage parasitic capacity (Cqi) formed between a first plate of the storage capacity and the reference conductor, the shift and storage parasitic capacities resulting in a loss of charge transmitted to the cell. Each cell comprises charge injection means (Cdinji, Cminji), the quantity of charge injected into a cell permitting partial or total compensation, or overcompensation, of the quantity of charge lost by the cell.

[0011] According to a first variant of the invention, the means for injecting charge into the cell comprise an injection capacity formed by a first electrode placed above a second plate of the storage capacity, a voltage equal to the inverse of the switched voltage is applied to the first electrode, and a voltage equal to the switched voltage is applied to the reference conductor.

[0012] According to a second variant of the invention, the means for injecting charge into the cell comprise an injection capacity formed by a second electrode placed above a second plate of the shift capacity, a voltage equal to k times the switched voltage being applied to the second electrode, k being a real number greater than 1, and a voltage comprised between 0 V and n times the supply voltage being applied to the reference conductor.

[0013] According to a preferred embodiment of the invention, the charge pump is implemented using silicon-on-insulator technology, more commonly termed SOI technology. The shift (Cdi) and storage (Cmi) capacities are then formed on an insulating layer which is itself formed on a silicon wafer. The reference conductor is then constituted by the silicon wafer which is in contact with the insulating substrate.

[0014] A charge pump according to the invention advantageously permits the production of electronic circuits of small dimensions (typically several mm²) and of high energy yield.

BRIEF DESCRIPTION OF THE FIGURES

[0015] Other characteristics and advantages of the invention will become apparent on reading a preferred embodiment with reference to the accompanying figures, among which:

[0016]FIG. 1 shows a schematic circuit diagram of a charge pump according to the prior art;

[0017]FIG. 2 shows the actual circuit diagram of the charge pump whose schematic circuit diagram is shown in FIG. 1;

[0018]FIG. 3 shows a shift capacity and a storage capacity of a charge pump, formed using silicon-on-insulator technology;

[0019]FIG. 4 shows a structural element of a charge pump implemented using silicon-on-insulator technology according to a first embodiment of the invention;

[0020]FIG. 5 shows a circuit diagram of a charge pump according to the first embodiment of the invention;

[0021]FIG. 6 shows a circuit diagram of the cascading of charge pumps according to the first embodiment of the invention;

[0022]FIG. 7 shows a circuit diagram of a charge pump according to the first embodiment of the invention, in the case in which the DC voltage to be converted is a negative voltage;

[0023]FIG. 8 shows a structural element of a charge pump formed using silicon-on-insulator technology according to a second embodiment of the invention;

[0024]FIG. 9 shows a circuit diagram of a charge pump according to the second embodiment of the invention.

[0025] In all the figures, the same reference numerals denote the same elements.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0026]FIGS. 1 and 2 have been described previously, and there is thus no need to return to them.

[0027]FIG. 3 shows a storage capacity Cmi and a shift capacity Cdi formed by silicon-on-insulator technology, more commonly termed SOI technology.

[0028] Silicon-on-insulator technology leads to the formation of components by diffusion of materials doped into a slice of silicon (commonly termed “wafer”). A diffusion of doped material is performed, for example, over a thickness of 1 μm in a wafer 500 μm in thickness. The active portion of the component is thus situated at the surface of the wafer. An insulating layer is intercalated between the active portion of the component and the thick portion of the wafer likewise termed back face or “backface”.

[0029] Referring to FIG. 3, the capacities Cmi and Cdi formed by SOI technology are placed on an insulating layer 2, itself placed on a backface 1. The backface 1 is a conductive zone whose potential is generally a reference potential for the circuit. By construction, the formation of the storage capacity Cmi and shift capacity Cdi is accompanied by the presence of respective parasitic capacities Cpi and Cqi. The storage parasitic capacity Cpi is formed by a plate of the storage capacity Cmi, the insulating layer 2 and the wafer 1. Likewise, the shift parasitic capacity Cqi is formed by a plate of the shift capacity Cdi, the insulating layer 2 and the wafer 1.

[0030] The presence of the parasitic capacities Cpi and Cqi leads to a loss of the charge transmitted to the cell. The charge loss phenomenon, of little importance if there are few cells, becomes very detrimental as the number of cells increases.

[0031] The yield ρ which measures the ability of a cell of useful capacity Cu to restore charges, and which is generally defined as the ratio of the amount of charge leaving to the amount of charge entering the cell, is given by:

ρ=(1−Cp/Cu),

[0032] where Cp is the parasitic capacity associated with the capacity Cu, Cp having a value very much smaller than Cu. The yield ρ is then generally less than 1.

[0033] The sensitivity of the transfer yield to the parasitic capacity is given by:

dρ/dCp=−1/Cu

[0034] The principle of the charge pump, which places in series n identical cells, implies that the voltage U_(i) provided by the cell of rank i is a function of the voltage U_(i−1) of the preceding cell of rank i−1, whence the expression:

Ui=ρ×U _(i−1)

[0035] To multiply by n the supply voltage Uo provided to the first cell in a charge pump, n cells in series are necessary. The output voltage Us is then the sum of all the elementary voltages, i.e.:

Us=Σ ^(n) ₁ U _(i), or

US=ρ.Uo+ρ² .Uo+ . . . +ρ ^(n) .Uo, or

Us=(ρ−ρ^(n+1))/(1−ρ).Uo, and thus

Us=η×Uo

[0036] The supply voltage Uo is thus multiplied by the coefficient η such that:

η=(ρ−ρ^(n+1))/(1−ρ),

[0037] the coefficient η being less than the number n of cells. Thus from this:

Us<n×Uo.

[0038] Thus, for example, a pump with 20 cells of unit yield 0.95 does not permit envisaging a voltage multiplication greater than 12 times the supply voltage.

[0039] For a high voltage application which is to deliver, for example, a voltage of 500 V starting from a supply voltage Uo of 2.5 V, 200 cells are theoretically necessary. The parasitic capacities represent about 2% of the useful capacity, and the unit yield p is then written:

ρ=0.98

[0040] The value of the output voltage is then Us such that:

Us=48×2.5 V, or

Us=120 V,

[0041] which is very far from the desired value.

[0042] A charge pump according to the known art is thus not capable of delivering an elevated voltage. Modifications to the structure are necessary. FIG. 4 shows a first type of structural modification of a charge pump implemented using silicon-on-insulator technology according to the invention.

[0043] A metallic screen 3 (electrode) is placed above the memory capacity Cmi, which is then comprised between the metallic screen 3 and the backface 1. A supply voltage Ucinv equal to the inverse of the switched voltage Uc is applied to the metallic screen 3, while the switched voltage Uc is itself applied to the backface 1.

[0044] The screen 3 thus supplied permits charge exchange with the capacity Cmi. This charge exchange is manifested, between the electrode 3 and the capacity Cmi, by a charge injection opposed to the charge loss which occurs between the capacity Cmi and the backface.

[0045] The electrode 3 and the plate of the capacity Cmi facing the electrode 3 define an injection capacity Cminji. The quantity of charge injected can partially or completely compensate, or even overcompensate, the lost quantity of charge. A total compensation between charge loss and charge injection is obtained when the following condition is satisfied:

Cminji=Cmi

[0046]FIG. 5 shows a circuit diagram of a charge pump according to the first embodiment of the invention. An inverter Inv receives at its input the switched voltage Uc and delivers an inverted switched voltage Ucinv. The inverted switched voltage Ucinv is applied to the injection capacities Cminji (i=1, 2, . . . , n), while the switched voltage Uc is applied to the backface.

[0047]FIG. 6 shows a circuit diagram of the cascading of two charge pumps implemented according to the first embodiment of the invention. A first charge pump switches a first low DC voltage Uo and a second charge pump switches a second low DC voltage Ul. The first charge pump delivers, for example, a DC voltage Ux equal to 1,000 V and the second charge pump delivers a DC voltage Uy equal to 500 V. The association of the two charge pumps thus leads to obtaining a voltage substantially equal to 1,500 V starting from the voltage Uo equal to, for example, 2.5 V.

[0048]FIG. 7 shows a circuit diagram of a charge pump according to the first embodiment of the invention, in the case where the DC voltage to be converted is a negative voltage.

[0049]FIG. 8 shows a structural element of a charge pump implemented using silicon-on-insulator technology according to a second embodiment of the invention.

[0050] A metallic screen 4 (electrode) is placed above the shift capacity Cdi, which is then comprised between the metallic screen 4 and the backface 1. A supply voltage Ua is applied to the metallic screen 4, and a supply voltage Ub is applied to the backface 1. The voltage Ua is a voltage equal to k times the switched voltage Uc, k being a real number greater than 1. The voltage Ua may thus, for example, be delivered from the voltage Uc by an amplifier having a gain greater than 1 or else may be a voltage taken from a node of one of the cells of the charge pump, k then being an integer greater than or equal to 2. The voltage Ub is a DC voltage comprised between 0 V and n×Uo.

[0051] The screen 4 thus supplied then permits exchanging charge with the capacity Cdi. This charge exchange is manifested by a charge injection, between the electrode 4 and the capacity Cdi, opposed to the charge loss between the capacity Cdi and the backface.

[0052] The electrode 4 and the plate of the capacity Cdi facing the electrode 4 define an injection capacity Cdinji. Here also, the quantity of charge injected can partially or completely compensate the quantity of lost charge. A total compensation between charge loss and charge injection is obtained when the following condition is satisfied:

Cdinji=Cdi.

[0053]FIG. 9 shows an example of a circuit diagram of a charge pump implemented according to the second embodiment of the invention. An amplifier A receives at its input the switched voltage Uc and delivers an amplified switched voltage equal to k×Uc. The amplified switched voltage is applied to the injection capacities Cdinji (i=1, 2, . . . , n).

[0054] It was mentioned above that the quantity of injected charge may partially or totally compensate for the quantity of lost charge. Advantageously, whatever the embodiment of the invention, it is likewise possible to produce an overcompensation of the charge loss by injecting more charge than is lost, for example to compensate for losses by Joule effect. The voltage provided at the output of the charge pump may then be greater than n times the supply voltage Uo. The transfer yield p is then written:

ρ=1−(Cp−Ci)/Cu,

[0055] where Ci represents the injection capacity (Cminji, Cdinji) and Cu the parasitic capacity (Cpi, Cqi).

[0056] In the case in which Ci=Cp (case of total compensation), the yield is equal to 1 and the output voltage Us is simply written:

Us=n×Uo

[0057] In general, according to the invention, the voltage Us can advantageously be adjusted solely by the design of the technological layers of the injection capacity Ci (Cminji, Cdinji).

[0058] Moreover, suppose there is an error E in one injection capacity Ci. Then:

Ci=(1+ε)×Cp.

[0059] The sensitivity of the transfer yield to the parasitic capacity is then written:

dp/dCp=d(1−(Cp−Ci)/Cu)/dCp, or

d(1+(ε.Cp)/Cu)/dCp=ε/Cu

[0060] As a result, the sensitivity of the yield of a charge pump according to the invention is advantageously ε times lower than the sensitivity of a charge pump according to the prior art. This characteristic advantageously has a favorable effect on the global energy yield of the charge pump. 

1. Charge pump comprising a block of n cells in series, each cell comprising a shift capacity (Cdi) and a storage capacity (Cmi), the voltage applied at the input of the block of n cells being a switched voltage (Uc) formed from a DC supply voltage (Uo), the shift capacity (Cdi) having a shift parasitic capacity (Cpi) formed between a first plate of the shift capacity and a reference conductor, and the storage capacity (Cmi) having a storage parasitic capacity (Cqi) formed between a first plate of the storage capacity and the reference conductor, the shift parasitic and storage capacities resulting in a loss of charge transmitted to the cell, characterized in that each cell comprises charge injection means (Cdinji, Cminji), the quantity of charge injected into a cell permitting partial or total compensation, or overcompensation, of the quantity of charge lost by the cell.
 2. Charge pump according to claim 1, characterized in that the means for injecting charge into the cell comprise an injection capacity (Cminji) formed by a first electrode (3) placed above a second plate of the storage capacity (Cmi), in that a voltage equal to the inverse (Ucinv) of the switched voltage (Uc) is applied to the first electrode (3), and in that a voltage equal to the switched voltage (Uc) is applied to the reference conductor.
 3. Charge pump according to claim 2, characterized in that it comprises an inverter (Inv) whose input is connected to the switched voltage (Uc) and whose output is connected to the set of first electrodes (3) of the n cells.
 4. Charge pump according to claim 1, characterized in that the means for injecting charge into the cell comprise an injection capacity (Cdinji) formed by a second electrode (4) placed above a second plate of the shift capacity (Cdi), in that a voltage equal to k times the switched voltage (Uc) is applied to the second electrode (4), k being a real number greater than 1, and in that a voltage (Ub) comprised between 0 V and n times the supply voltage (Uo) is applied to the reference conductor.
 5. Charge pump according to claim 4, characterized in that the second electrodes (4) of the n cells are connected to the same node of a cell of the charge pump to which a voltage is applied equal to m times the switched voltage (Uc), m being an integer greater than or equal to
 2. 6. Charge pump according to claim 4, characterized in that it comprises an amplifier (A) of gain greater than 1 whose input is connected to the switched voltage (Uc) and whose output is connected to the set of second electrodes of the n cells.
 7. Charge pump according to any one of the foregoing claims, characterized in that it is implemented using silicon-on-insulator technology such that the shift (Cdi) and storage (Cmi) capacities are formed on an insulating layer (2) which is formed on a wafer (1) constituting the reference conductor, the shift parasitic capacity (Cqi) being formed by a plate of the shift capacity, the insulating layer (2), and the wafer (1), and the storage parasitic capacity (Cpi) being formed by a plate of the storage capacity, the insulating layer (2), and the wafer (1). 